1. Field of the Invention
The present invention relates to a method of producing a molded latex product which does not require a vulcanizing step.
2. Description of Prior Developments
Latex or emulsion-polymerization technology is well-known in the art and includes vinyl acetate, vinyl chloride, acrylics, acrylonitrile and ethylene, in addition to the monomers of styrene, butadiene and isoprene. Emulsion polymerization is a heterogeneous noncatalytic reaction. The mechanistic reaction model is based on the mixing of a monomer and a surfactant in water to yield both monomer droplets and solubilized monomer in micelles. The free radical is generated in the aqueous phase and can enter either the monomer droplet or monomer-swollen micelle.
Generally, the course of batch-emulsion polymerization is subdivided into three intervals. Particle formation takes place during interval I and, depending on the water solubility of the monomer, normally ends at 1-5% conversion. Interval II is characterized by the growth of the primary particles and ends when the separate monomer phase disappears. In interval III, the particle volume decreases, i.e. the monomer concentration in the particle decreases with a corresponding increase in polymer viscosity.
Solubilization is the spontaneous passage of molecules of an insoluble substance in water into the interior of the micelle of a dilute surfactant solution. Solubilization involves the process of diffusion from a separate monomer phase to the micelle. Emulsification occurs beyond the monomer saturation point of micelles with further addition of monomer. There is a strong temperature effect on the solubilization of an oil in the aqueous solution of a nonionic surfactant that results from a change in the hydrophilic-lipophilic balance (HLB) of the surfactant.
Emulsion polymers generally are considered to be metastable systems. The large surface represented by the particles is thermodynamically unstable and any perturbation affecting the balancing forces results in a change of the kinetics of particle agglomeration. An interface, e.g. the latex polymer particle aqueous phase interface, may acquire an electrostatic charge by any one of several well-known mechanisms.
Freeze-thaw stability, which is important to shipping and storing of latices, may be accomplished in any one of several ways including the incorporation of ionogenic comonomers and the prevention of ice formation. This may be achieved by the addition of water-soluble solvents, such as ethylene glycol, propylene glycol, etc. which depress the freezing point. With ionogenically stabilized latices, the addition of a water-soluble organic solvent swells the latex particles but does not dissolve them The latex particles are stabilized by charge and hydration of polar surface groups. The degree of charge stabilization is determined by the balance between the surface-active agent concentration on the particle surface and particle size. The stability and the performance properties of latices depend in part on the particle size and distribution. Viscosity is dependent upon latex rheology.
The traditional raw material has been natural latex concentrate preserved with approximately 0.7% ammonia and containing about 60% of rubber. This is normally prepared on rubber estates by centrifugation of field latex. Creamed latex concentrate may also be used. Concentrates are available containing a lower amount (0.2%) of ammonia together with a secondary preservative. Blends of natural rubber latex with styrene-butadiene (SBR) latices are frequently employed. The choice of blend depends on factors such as price, processing behavior and technical requirements for the final product.
There are several well-known methods of producing latex products. Dipped articles, which are seamless hollow articles, in general are formed from polymer solutions or dispersions by completely coating a mold surface, then drying and cross-linking the coating. The molds are coated by dipping them briefly into the polymer solution or dispersion so that polymer precipitates on their surface. Latex products precipitated on molds are dried in hot air and vulcanized. Articles produced in hollow molds by casting may be considered a special category of dipped articles. In such cases, the precipitation of the polymer takes place mainly by absorption of the serum through the pores. Very high wall thicknesses are achieved in this way.
Molded foams are also well-known in the art. The greatest proportion of molded foam is produced by the Dunlop process. Here, an aqueous sodium silicofluoride dispersion is added to the readily mixed, vulcanizable latex compound during the mechanical foaming process. The quantity depends on the desired potlife of the foam compound, which is generally adjusted to 8-12 minutes, a time sufficient to fill the foam molds. After vulcanization, the articles are removed from the mold, washed and dried.
In the so-called Kaysam process sensitization is carried out with ammonium salts in the presence of zinc oxide whereas in the Dunlop process sodium or potassium silicofluoride is used with zinc oxide.
Polyvinyl methyl ether and polyoxypropylene glycol are easily soluble in water at room temperature, but at a certain temperature they become insoluble in water and are precipitated. Simultaneously they induce the latex particles to coagulate as an irreversible coagulum, resulting in a uniform film deposited on the surface.
In the case of heat sensitization with trypsin, the stabilizing protein substances still present in the latex are decomposed and the latex is so sensitized that, after the addition of zinc oxide and simultaneous heating of the compound to about 50.degree. C., spontaneous coagulation takes place in about 3 to 4 minutes.
The Talalay process is an alternative foam process by which the vulcanizable latex compound is frothed in a mechanical mixer to give a foam with relatively high density which is poured into molds. The closed molds are evacuated. The latex compound foams to its final density because of the vacuum applied and thus fills the mold completely. By cooling the mold to about 30.degree. C. the structure of the foam is fixed. Carbon disxide is led into the mold, thus lowering the pH of the frozen foamed latex compound, which then gels. The foam can then be thawed and the mold heated in stages to 110.degree. C. After vulcanization is complete, the article is removed from the mold, washed and dried.
In each of the above processes, a synthetic rubber latex is employed. Synthetic latices are differentiated from other polymer dispersions [i.e. poly(vinyl chloride), ethylene vinyl acetate copolymer or polyacrylates]. The criterion for identifying a synthetic latex is recognized to be the irreversible conversion of the polymer from the thermoplastic to the elastomeric state by vulcanization.
Latex vulcanizates are dried in air room temperature or at an elevated temperature. The drying of very thin sectioned goods (0.5-0.1 mm) is extremely rapid. The drying of thick-walled goods takes place in two phases, the first phase being rapid but the second phase taking considerably longer owing to the reduced water permeability of the film. Therefore, hot air with a relative humidity of 50% to 60% is often passed over the drying films. The humidity in the hot air prevents the surface of the rubber film from closing as rapidly as it would otherwise do and thus enables water inside the film to reach the surface more easily. At an elevated temperature, i.e. 100.degree. C. to 110.degree. C., the drying takes place rapidly. It is generally considered advisable to dry at a moderate temperature initially (about 70.degree. C.) and then to pass from drying to curing by increasing the temperature to 110.degree. C.
The method of the present invention, which may be used with any type of latex, produces a superior latex product which does not require vulcanization and, therefore, does not require the drying of latex vulcanizates. Surprisingly, the present method teaches the production of solid latex foam without requiring a vulcanization step.